This paper presents Forecast-Aided Distribution System State Estimation algorithm based on the Unscented Kalman Filter. It uses pseudo-measurements based on the forecasted load profiles and previous state estimation, in conjunction with optimally placed limited number of load active and reactive power measurements. Presented estimation results are obtained by simulations on a 33 node test system using forty one day hourly load data. Results obtained using a test system are showing that this algorithm can make good estimate of the voltage amplitude even with high error of the forecasted loads.